Portable electronic devices such as a laptop personal computer (laptop PC), a PDA (Personal Digital Assistant), and a mobile phone may carry a battery pack that includes a secondary battery, often termed a storage battery. The storage battery supplies power to a system load of the electronic device, and is typically charged by a charger mounted in a main body of the electronic device. A charge/discharge circuit inside the battery pack is provided with a protection element formed of a low melting point metal material for disconnecting the storage battery from the charge/discharge circuit when an abnormal condition occurs in the storage battery during charge or discharge.
Japanese Patent Application Laid-Open No. 2007-123129 discloses a protection element capable of reliably blowing a fuse by a heating resistor even when a voltage of a battery included in a battery pack varies widely. This protection element includes a low melting point metal that is fused when heated and a heating resistor that is thermally coupled to the low melting point metal and heats the low melting point metal with Joule heat generated by a current applied to the heating resistor. The heating resistor is connected with a switch that is turned on to cause the current to flow when an abnormal condition occurs in the storage battery. Japanese Patent Application Laid-Open No. 2000-340267 describes a constant current circuit for controlling a current flowing through a heater element within a fixed range. Japanese Patent Application Laid-Open No. 2006-269437 describes a protection element including one heater element connected to a central terminal of a low melting point metal body.
Typically, a charge/discharge circuit in a battery pack is provided with a return-type protection circuit that maintains safety of the battery pack by turning a switch off upon detecting that an abnormal condition occurs in the battery pack during charge or discharge. The return-type protection circuit allows for return if the abnormal condition of the battery pack is removed after the operation. Since high safety is required of the battery pack, a non-return-type protection element (hereafter simply referred to as a protection element) that interrupts the charge/discharge circuit by fusing a low melting point metal with Joule heat is also provided as a backup in the case where the return-type protection circuit does not operate properly. The protection element serves as a final protection device when the return-type protection circuit does not operate properly, and therefore is required to operate with high stability and reliability.